1. Field of the Invention
The invention relates generally to seismic prospecting. More particularly, the invention relates to marine seismic sources for generating seismic waves.
2. Background of the Technology
Scientists and engineers employ seismic surveys for exploration, archeological studies, and engineering projects. In general, a seismic survey is an attempt to map the subsurface of the earth to identify formation boundaries, rock types, and the presence or absence of fluid reservoirs. Such information greatly aids searches for water, geothermal reservoirs, and mineral deposits such as hydrocarbons (e.g., oil, natural gas, etc.). Petroleum companies frequently use seismic surveys to prospect for subsea petroleum reserves.
During a subsea or marine seismic survey, an acoustic energy source, also referred to as a seismic energy source or simply a seismic source, is introduced into the water above the geologic formations of interest. Each time the source is triggered, it generates a seismic energy signal that propagates downward through the water and the water-sea floor boundary, and into the subsea geological formations. Faults and boundaries between different formations and rock types create differences in acoustic impedance that cause partial reflections of the seismic waves. These reflections cause acoustic energy waves to move upward and out of the formation, where they may be detected at the seafloor by an array of seismic energy receivers (e.g., ocean-bottom geophones), or where they may be detected within the seawater by an array of seismic energy receivers (e.g., spaced hydrophones).
The receivers generate electrical signals representative of the acoustic or elastic energy arriving at their locations. The signals are usually amplified and then recorded or stored in either analog or digital form. The recording is made as a function of time after the triggering of the seismic energy source. The recorded data may be transported to a computer and displayed in the form of traces, i.e., plots of the amplitude of the reflected seismic energy as a function of time for each of the seismic energy receivers. Such displays or data subsequently undergo additional processing to simplify the interpretation of the arriving seismic energy at each receiver in terms of the subsurface layering of the earth's structure. Sophisticated processing techniques are typically applied to the recorded signals to extract an image of the subsurface structure.
There are many different methods for producing acoustic energy waves or pulses for seismic surveys. Conventional seismic surveys typically employ artificial seismic energy sources such as explosives (e.g., solid explosives or explosive gas mixtures), shot charges, air guns, or vibratory sources to generate acoustic waves. Some of these approaches provide for strong acoustic waves, but may be harmful to marine life and/or be incapable of generating energy only within a specified frequency range of interest. A more controllable approach is the use of a subsea or marine reciprocating piston seismic source. Traditionally, such devices utilize a piston that reciprocates against the water to generate extended-time, acoustic-energy frequency sweeps. The piston is driven by a source of mechanical force, which may be a linear actuator, a voice coil, or a piezoelectric crystal transducer. The piston may be directly driven, with the motion of the piston almost entirely constrained, or may resonate by balancing water forces against a tunable spring, with the driving force only “topping up” the energy lost to the water. Further, the piston may be partially constrained and partially allowed to undergo a controlled resonance. The tunable spring may be, for example, a mechanical spring, a regenerative electromagnetic inductive device, an air spring, or a combination of these.
FIG. 1 shows a simplified example of a conventional reciprocating piston marine seismic source 10 disposed beneath the sea surface 11 in water 12. Source 10 includes a cylinder 15 having a central axis 19 and a piston 20 coaxially disposed in the cylinder 15. Cylinder 15 has a lower end 15a open to the water 12 and an upper end 15b closed off with a cap 16. Piston 20 sealingly engages the inner surface of cylinder 15, thereby defining a chamber or volume 17 within cylinder 15 that is filled with a compressible gas such as air or nitrogen. Piston 20 has a flat or planar end 20a that faces and operates against the water 12 in lower end 15a of cylinder 15 and a flat or planar end 20b opposite end 20a that faces chamber 17. Piston 20 is coupled to an actuator 25 disposed in chamber 17 with a shaft 21. Actuator 25 is fixed relative to cylinder 15 with supports 26, and axially reciprocates piston 20 within cylinder 15. As piston 20 reciprocates, planar face 20a acts against water 12 in lower end 15a to generate acoustic energy waves that propagate through the water 12.
As shown in FIG. 1, in many conventional oscillating piston marine-seismic sources, the piston 20 has a planar surface 20a that faces and operates against the water 12. In addition, the piston 20 is completely disposed within cylinder 15. In particular, cylinder open end 15a extends axially beyond planar face 20a of piston 20, thereby defining a water-filled recess or cavity 21 at open end 15a of the cylinder 15. Computational-fluid-dynamics (CFD) modeling has indicated that for relatively low frequencies (e.g., less than 5 Hz) and large amplitudes (e.g., greater than 200 mm of peak-to-peak amplitude) of piston movement, planar surface 20a and water-filled recess 21 at the open end 15a of most conventional piston-driven seismic sources combine to produce undesirable turbulence in the water 12 proximal open end 15a. For example, in FIG. 2, CFD modeling illustrates velocity vectors in the water 12 around the open end 15a of the cylinder 15. In particular, turbulent vortices 13, 14 are generated in the water 12 within and outside, respectively, the open end 15a of the cylinder 15. Such turbulence and associated vortices undesirably cause a portion of the energy generated by the piston 20 and transferred into the water 12 to be dissipated uselessly into heat, thereby reducing the potential acoustic energy and the overall acoustic efficiency of the device 10.
At higher frequencies and shallow water depths, another serious problem can happen with an oscillating-piston seismic source—cavitation. In general, cavitation occurs when the local static pressure head minus the local vapor pressure head becomes less than the local piston-velocity head for some point on the piston face. When cavitation occurs, the seawater temporarily decouples from the moving piston face, leaving a vacuum or vapor bubble adjacent to that part of the piston face. The vacuum then collapses violently, possibly damaging the piston face in the process. The collapse also produces a violent impulsive sound, the avoidance of which is at least one reason to use an oscillatory piston source. Still further, the abrupt collapse produces turbulence, which dissipates energy uselessly as heat instead of as acoustic radiation.
Accordingly, there remains a need in the art for marine seismic sources that produce energy in a controlled frequency sweep that is extended in time, without any impulsive shocks, and that produce energy only in the frequency bands of interest, and not outside it, so that only the minimum necessary peak power is emitted at each frequency and all the energy emitted is useful. Such sources would be particularly well received if they can produce energy at frequencies lower than about 8 Hz, which has proven to be difficult to achieve to date using conventional air guns.